As a writer, you might not notice repeats, but they set off alarm bells in your readers' heads: "Didn't they just say that?" Repeated long phrases occur when you use the same three or more word phrase within a few paragraphs. You can fix this error by rewriting the duplicate instances of using the same phrase.
Many writers aren't able to identify repeated words and phrases in their own work. That's because the phrase might sound natural to you, since you were the one who chose it in the first place.
Similarly, during the editing phase, you often read the same passages over and over to make changes. That can make you impervious to that echo feeling your readers get.
Reading your work out loud can help you identify repeated words and phrases. When you read your work out loud, you pay more attention to the words you're reading, so you're more likely to notice repeats.
You can also use ProWritingAid to identify repeats. The Repeats Check highlights repeated phrases that occur within a few paragraphs so you can track down unintended repetition and edit as necessary.
It's up to you as the author to decide how you want to fix repeated phrases. In most instances, you'll want to rewrite the phrase with different words. In some cases, though, you might want to keep the repeat in, such as if it's a phrase where the order or specific words can't be changed. Use your discretion to make changes to instances of repeats.